1. Field of the Invention
This invention relates to a magnetic pole position detector for an electric motor, and particularly to a magnetic pole position detector for an electric motor with multipolar rotors.
2. Description of the Prior Art
Rotary motion of a step motor is regulated by a frequency of input pulses for controlling the motor. However, when a high load is applied to the motor, or the motor is rapidly accelerated and/or the motor is running at a high rotational speed, it often occurs that the motor steps out and an abnormal vibration is caused to the rotation of the rotor, since the rotor becomes unable to correctly follow the command pulse.
Recently, a closed-loop drive of a step motor has been adopted and the cause of the step out has been reduced to a certain degree, however, the problem of the step out has not been settled yet in principle.
If a step motor can work as a brushless DC motor to avoid the step out of the motor as mentioned above, then the advantages of both the step motor and the brushless DC motor can be utilized and the stable rotation of the step motor can be held even at the starting of the motor or at rapid changes of the load being applied.
In order to make a step motor work as a brushless DC motor, it is necessary to detect the magnetic pole positions of the rotor of the step motor and to supply commutation signals that give a proper electrical angle to the motor winding.
Many methods to make a hybrid step motor work as a brushless DC motor have been proposed in which an encoder is attached directly to a rotation axis of the motor, the encoder signal is synchronized with the magnetic pole position of the rotor of the motor and the synchronized encoder signal is used as the commutation signal.
Further, methods for detecting a position of a multipolar rotor using low-priced Hall effect devices instead of a high-priced encoder are disclosed in Unexamined Patent Publication Nos. 7-174583 and 9-201206 for examples.
In order to lower the vibration of a motor, it is advantageous to make the step angle of the rotor very narrow. However, if the step angle of the rotor is made very narrow, then the number of magnetic poles of the motor as a brushless DC motor will become excessively large. For example, a three-phase step motor with a step angle of 0.60 has 200 magnetic poles. In order to make a step motor work as a brushless DC motor, it is necessary to detect the magnetic pole positions of 200 magnetic poles with high accuracy.
Encoder signals are utilized as commutation signals in the above mentioned system comprising a step motor and an encoder. Accordingly, it is necessary to make the output pulse position of the encoder coincide with the magnetic pole position of the rotor of the motor. However, in case that a number of magnetic poles exist, positioning of the encoder with the axis of the rotor with precise mechanical angle is quite difficult when the former and the latter are connected with each other. Further, fine adjustment of the positioning requires a long time even for a motor with relatively larger step angle.
In the above mentioned invention disclosed in the Unexamined Patent Publication No. 7-174583 in which Hall effect devices are used, a magnetic drum having magnetic poles of which number is equal to that of a step motor is fixed to the axis of the motor, magnetism is collected by two pieces of sensor cores arranged with an angle between them, and the variation in magnetic flux is detected by the Hall effect devices.
Also in this method, it is necessary to make the position of the magnetic pole of the magnetic drum coincide with the magnetic pole position of the rotor of the motor. Accordingly, the above mentioned problem concerning the precision of the positioning in mechanical angles remains unsolved. Further, positioning of two pieces of sensor cores is annoying.
Further, in the above mentioned invention disclosed in the Unexamined Patent Publication No. 11-046498, since multipolar magnet is also required, positioning of the multipolar magnet with the rotor of the motor is unavoidable.
It is an object of the present invention to solve the above mentioned problems and to provide a magnetic pole position detector for an electric motor which is capable of obtaining commutation signals with high accuracy at a low price.
According to a preferred embodiment of the present invention, there is provided a magnetic pole position detector for an electric motor comprising a detection rotor, a detection stator, magnetic flux concentration tips and magneto electro transducers,
the detection rotor is fixed to an axis of rotation of the electric motor and comprised of a pair of disc-like rotors having a plurality of first toothlike protrusions on their circumferences and a permanent magnet of which N-S pole is in the axial direction of the electric motor and the diameter of which is smaller than those of the disc-like rotors,
the disc-like rotors are assembled concentrically with each other so as to make respective first toothlike protrusions coincide with each other in the axial direction,
the permanent magnet is fixed concentrically between the disc-like rotors,
the detection stator is comprised of a pair of stator cores,
the stator core is comprised of an annular portion and a plurality of stator teeth for collecting magnetic flux flowing through the first toothlike protrusions of the detection rotor,
the stator teeth are formed at regular intervals on the inner circumference of the annular yoke, each of the stator teeth has inner end portion on which a plurality of second toothlike protrusions are formed,
the stator cores are assembled concentrically with each other with a gap in the axial direction between them so as to make respective second toothlike protrusions of the stator teeth coincide with each other in the axial direction,
the magnetic flux concentration tips are placed in the gaps in the axial direction for concentrating magnetic flux in the gaps in the axial direction,
the magneto electro transducers are placed in the gaps in the axial direction for transforming the magnetic flux into electrical signals,
the detection rotor and the detection stator are assembled so as to make the first toothlike protrusions of the detection rotor and the second toothlike protrusions of the stator teeth of the detection stator oppose with each other with a gap in the radial direction of the electric motor between them,
wherein the magnetic pole position of the electric motor is detected by measuring the magnetic flux flowing through the stator teeth that varies in accordance the rotation of the detection rotor.
The magnetic pole position detector for an electric motor according to the present invention makes it possible to accurately detect a magnetic pole position of a rotor of a hybrid step motor with minute step angles when the motor works as a brushless DC motor. The magnetic pole position detector for an electric motor according to the present invention has simple structure easy to be constructed at a low price.
An motor equipped with the magnetic pole position detector for an electric motor according to the present invention can work with less fluctuation in rotational speed, with enlarged operating range and with higher operational efficiency, and thus the scope of its use can be expanded.